Override blend mode, FPS sparkline, fix api_input persistence

New features:
- Override composite blend mode: per-pixel alpha from brightness
  (black=transparent, bright=opaque). Ideal for API input over effects.
- API input test preview FPS chart uses shared createFpsSparkline
  (same look as target card charts)

Fixes:
- Fix api_input source not surviving server restart: from_dict was
  still passing removed led_count field to constructor
- Fix composite layer brightness/processing selectors not aligned:
  labels get fixed width, selects fill remaining space
- Fix CSPT input selector showing in non-CSPT CSS test mode
- Fix test modal LED/FPS controls showing for api_input sources
- Server only sends test WS frames when api_input push_generation changes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/api/schemas/color_strip_sources.py

@@ -31,7 +31,7 @@ class CompositeLayer(BaseModel):
     """A single layer in a composite color strip source."""
 
     source_id: str = Field(description="ID of the layer's color strip source")
-    blend_mode: str = Field(default="normal", description="Blend mode: normal|add|multiply|screen")
+    blend_mode: str = Field(default="normal", description="Blend mode: normal|add|multiply|screen|override")
     opacity: float = Field(default=1.0, ge=0.0, le=1.0, description="Layer opacity 0.0-1.0")
     enabled: bool = Field(default=True, description="Whether this layer is active")
     brightness_source_id: Optional[str] = Field(None, description="Optional value source ID for dynamic brightness")